Subnetting Help with prefix

Sapling09 · Jan 31, 2014

Ok guys today I had a CCNA test and I have completely screwed up the subnetting.

Lets say I have address
10.0.208.0/20
and network A has 1023 hosts and network B 253 hosts

The subnetting would look something like this

1023 | 10.0.208.0/21| 10.0.208.1 - 10.0.215.254| 10.0.215.255
253 | 10.0.216.0 | 10.0.216.1 - 10.0.216.254| 10.0.216.255

Lets forget about the subnet masks for now

1023 fits in 2048 which has prefix of 22.

I do not understand why is prefix in the first adress /21 instead of /22. I did this error in the test and I can not understand what I got wrong.

If you write down the prefixes.

4096 2048 1024 512 256 128 64 32 16 8 4 2 1

Then using 4096 is a complete waste of adresses isn't it?

Please explain to me.

boosted1g · Jan 31, 2014

I have been going through your post trying to figure out where exactly you went wrong, where is what I can find.

First of 2048 is prefix 21, not 22 (1=32, 2=31,4=30...32=27...512=23, 1024=22, 2048=21.

The reason why 1023 can not be /22 is because /22 = 1024 but when you subtract the 2 it is now 1022, thus 1023>1022 and thus 1023 would be /21.
Now since you only needed 1 IP address to complete your network A, you have 253 availible IP address of your /20 IP block for network B.

Instead of attempting to explain the whole process in the thread check this out:
http://www.professormesser.com/n10-005/subnetting/

Sapling09 · Jan 31, 2014

Ok I kind of understand almost everything except.

128 has prefix of 25
256 has prefix of 24

You said 1024 is 22

But I do not understand what does change between 256 and 1024

Because, prefixes from 1 - 256 are symetric, for example. 1 has prefix of 32 and you can keep substracitng 1 till you reach 256 thus giving you prefix 24. But if you substract 1 one more time you should get prefix 23 for 1024....

I am kind of confused now...

bill001g · Jan 31, 2014

It depends how you look at this. The number exactly doubles for ever bit you reduce the mask.

If you want a math formula you can use 2^(32-length of prefix). 2^(32-22) ---> 2^10--> 1024

It is easier to memorize the table in most cases

boosted1g · Jan 31, 2014

128 prefix 25
256 prefix 24
512 prefix 23
1024 prefix 22

Sapling09 · Jan 31, 2014

Oh yes I understand now me dumb.

I have completely forgotten about 512. That is why 1024 had prefix 23 instead of 22.

Oh yes. thanks for the help guys.

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